drm/i915/guc: Skip cleaning up the doorbells on error-before-allocate

If we fail the module load, we may try and cleanup before we even
allocate the GuC clients. KISS in order to try and re-enable
drv_module_reload for BAT.

Testcase: igt/drv_module_reload/basic-reload-inject
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180712105830.20390-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_guc_submission.c

@@ -910,8 +910,12 @@ static void guc_clients_doorbell_fini(struct intel_guc *guc)
 		__update_doorbell_desc(guc->preempt_client,
 				       GUC_DOORBELL_INVALID);
 	}
-	__destroy_doorbell(guc->execbuf_client);
-	__update_doorbell_desc(guc->execbuf_client, GUC_DOORBELL_INVALID);
+
+	if (guc->execbuf_client) {
+		__destroy_doorbell(guc->execbuf_client);
+		__update_doorbell_desc(guc->execbuf_client,
+				       GUC_DOORBELL_INVALID);
+	}
 }
 
 /**